This invention relates generally to an apparatus for detecting coupling joints in subsurface tubing or casing set within a well bore and, more specifically, to electronic circuitry for processing electrical signals indicative of the coupling joints.
In the process of completing most oil and gas wells, a string of conduit or "casing" is placed into the earth borehole and cemented into place by pumping a slurry of cement between the borehole wall and the casing string. The casing is run into the borehole in various length sections with the diameter of the casing determined by the size at which the borehole is drilled. The sections of casing are joined into a string by a variety of threaded methods referred to generally as "collars".
Once casing is cemented into place within the borehole, the casing can be perforated at the suspected production zones as determined by commonly used well logging methods. When traversing the wellbore with a perforating instrument, it is desirable to have a quick, easy and reliable method to monitor the location of the perforating device in the borehole so that it can be aligned precisely opposite the formation to be perforated. To provide an accurate position determination, the operator must be able to correlate the depth of the perforating instrument with previously run well logs. This correlation is made easier by detecting when the instrument is proximate a specific casing collar. Such a determination is made using a casing collar indicating instrument.
Casing collar indicator instruments used in the industry typically emit a magnetic field by means of a permanent magnet or an oscillator circuit coupled to an electrical coil. Changes in the magnetic field resulting from variations in the casing mass caused by coupling joints are detected by the casing collar instrument and an electrical signal is transmitted to signal recovering circuitry in the form of either a shifting d.c. voltage signal or a sine wave signal of a predetermined frequency. Typically, the signal processing system had to have electronic circuitry for processing the d.c. signal and separate electronic circuitry for processing the a.c. signal. Further, the a.c. signal processing circuitry has been found to be less than fully reliable due to mistriggering. The most commonly used a.c. signal processing circuitry comprises a bandpass filter designed to pass the collar indicator instrument output frequency and attenuate other frequencies. It is not uncommon for this frequency dependent design to suffer from false triggering when a pulse repetition rate of approximately the same frequency as the collar indicator instrument sine wave signal is at the input or a large pulse at the input causes the bandpass filter to ring at its resonant frequency.
These and other disadvantages are overcome with the present invention by providing new and improved signal processing circuitry capable of handling either d.c. or a.c. collar indicator signals.